This chapter discusses the manufacture of important industrial chemicals like caustic soda, sodium chloride, soda ash and chlorine gas. It explains the electrolysis process used to produce caustic soda and chlorine as coproducts from salt. Sodium chloride is obtained from salt mines and purified through processes like solar evaporation or vacuum pans. The major industrial process to produce soda ash is the Solvay process which uses common salt, limestone and ammonia as raw materials.

2. • I referred this chapter “Alkali and chlorine” as it was under our cec assignment of
industrial chemistry 2021 by our industrial chemistry faculty Dr Viraj Bhanvadia sir,
Assistant Professor.
• This chapter mainly focuses on the manufacture of caustic soda, sodium chloride, soda
ash and chlorine gas. It has explained it thoroughly and used diagrams about the
processes in unembellished words which is beneficiary for students but it would be great
if it has more pictures related to the topics.
• The manufacture of caustic soda, soda ash and chlorine is one of the most important
heavy chemical industries. Caustic soda, sodium carbonate and chlorine are extensively
used in large number of industries. Their applications are so widespread that hardly a
consumer product is sold that is not dependent at some stage of its manufacture on
chlorine and alkalies.

3. SODIUM CHLORIDE
• SODIUM CHLORIDE COMMONLY KNOWN AS SALT , IS AN IONIC COMPOUND WITH THE CHEMICAL
FORMULA NACL WITH THE RATIO OF 1:1 OF SODIUM AND CHLORINE. A WIDELY DISTRIBUTED INORGANIC
COMPOUND THROUGHOUT THE WORLD AS IT IS VITAL FOR THE GROWTH OF LIVING BEINGS.
• IN INDIA, ABOUT 70% OF THE SALT IS CONSUMED BY HUMAN BEINGS AND THE REST 30% IS USED IN THE
MANUFACTURE OF ALKALIS AND OTHER INDUSTRIAL PRODUCTS SUCH AS SOAPS, TEXTILES, DYES ETC.
THE MAIN SOURCES OF COMMON SALT IN INDIA; SEA WATER, SALT LAKES, SUBSOIL & ROCK SALT.
• SALT IS THE BASIC RAW MATERIAL FOR THE MANUFACTURE OF CAUSTIC SODA AND CHLORINE, SODA ASH
(SODIUM CARBONATE), SODIUM SULPHATE, HYDROCHLORIC ACID ETC. SALT IS ALSO USED IN A LARGE
NUMBER OF OTHER INDUSTRIES, SUCH AS HYDROGENATION OF OIL, MANUFACTURE OF SOAP, DYES,
TEXTILE, FOOD PROCESSING ETC.
Properties
Density 2.17 g/cm
Molar mass 58.443 g/mol
Appearance Colorless cubic
crystals
Odor Odorless

4. MANUFACTURING OF SODIUM CHLORIDE
SALT OBTAINED FROM SOURCES IS IN SOLUTION OR LIQUID FORM. THIS FORM IS CALLED AS
BRINE. THE METHODS FOR CONCENTRATING THE BRINE SOLUTIONS ARE:
• SOLAR EVAPORATION- IT IS OLDEST, CHEAPEST AND BEST MANUFACTURING METHOD OF
SALT WHICH IS WIDELY USED IN INDIA. SOLAR SALT PRODUCTION IS, TYPICALLY, THE CAPTURING OF SALT
WATER IN SHALLOW PONDS WHERE THE SUN EVAPORATES MOST OF THE WATER. THE CONCENTRATED
BRINE PRECIPITATES THE SALT WHICH IS THEN GATHERED. ANY IMPURITIES THAT MAY BE PRESENT IN THE
BRINE ARE DRAINED OFF AND DISCARDED PRIOR TO HARVESTING.
• ARTIFICIAL EVAPORATION- THIS METHOD IS FOR THE COUNTRIES WHERE THE SOLAR
EVAPORATION IS NOT POSSIBLE IN LARGE SCALE. METHOD OF SALT PRODUCTION USED BY MORTON SALT
IS THE EVAPORATION OF SALT BRINE BY STEAM HEAT IN LARGE COMMERCIAL EVAPORATORS, CALLED
VACUUM PANS. THIS METHOD YIELDS A VERY HIGH PURITY SALT, FINE IN TEXTURE, AND PRINCIPALLY USED
IN THOSE APPLICATIONS REQUIRING THE HIGHEST QUALITY SALT. THE FIRST PART OF THE OPERATION IS
KNOWN AS SOLUTION MINING. WELLS ARE DRILLED FROM SEVERAL HUNDRED TO 1,000 FEET APART INTO
THE SALT DEPOSIT. THESE WELLS ARE CONNECTED VIA LATERAL DRILLING, A RECENTLY DEVELOPED
TECHNOLOGY. ONCE THE WELLS ARE CONNECTED, THE SOLUTION MINING OPERATION BEGINS: WATER IS
PUMPED DOWN ONE WELL, THE SALT BELOW IS DISSOLVED, AND THE RESULTING BRINE IS FORCED TO THE

5. THIS SERIES OF VACUUM PANS OPERATES ON A VERY SIMPLE PRINCIPLE: WHENEVER PRESSURE IS LOWERED, THE
TEMPERATURE
AT WHICH WATER WILL BOIL IS ALSO LOWERED. FOR INSTANCE, UNDER NORMAL AIR PRESSURE AT SEA LEVEL,
WATER BOILS AT
212°F. BUT AT TEN THOUSAND FEET ABOVE SEA LEVEL, WHERE AIR PRESSURE IS MUCH LESS, WATER BOILS AT 194°F.
VACUUM PANS MAY OPERATE AT AS LOW AS 100°F. IN THE VACUUM PAN PROCESS, STEAM IS FED TO THE FIRST
PAN. THIS CAUSES THE BRINE IN THE PAN TO BOIL. THE STEAM FROM THE BOILING BRINE IS THEN USED TO HEAT
THE BRINE IN THE SECOND PAN. THE PRESSURE IN THE SECOND PAN IS LOWER, ALLOWING THE STEAM MADE BY
THE BOILING IN THE FIRST PAN TO BOIL THE BRINE IN THE SECOND PAN. THE PRESSURE IS REDUCED STILL FURTHER
IN EACH SUCCEEDING PAN. THIS ALLOWS THE STEAM MADE BY THE BOILING BRINE IN THE PREVIOUS PAN TO BOIL
THE BRINE IN THE NEXT PAN. WHILE THE BOILING OPERATION COULD BE DONE WITH JUST ONE PAN, SEVERAL PANS
IN A ROW PRODUCE MORE SALT PER POUND OF STEAM, THUS ALLOWING GREATER ENERGY EFFICIENCY.
• FREEZING METHOD-SALT IS ALSO MANUFACTURED BY FREEZING THE BRINE.

6. • Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula
NaOH. It is a white solid ionic compound consisting of sodium cations Na+ and hydroxide anions OH−
• Caustic soda is an important heavy chemical and occupies among the basic chemicals a position
equal in importance to sulphuric acid and ammonia. It is used in soap, rayon, dyes, paper, drugs,
and foods. rubber, textiles, chemicals, bleaching, metallurgy and petroleum industries.
• Electrolysis – This process produces 2.25 tonnes of 50% caustic soda with each ton of chlorine. The
primary raw material is common salt, usually in the form of underground deposits.
Note- Caustic soda and chlorine are produced almost entirely as coproducts by the electrolysis of brine. The
process accounts for 80% caustic soda and more than 95% of chlorine production in India. In the electrolysis of
brine, chlorine is liberated at the anode and caustic soda together with hydrogen is produced at the cathode.
 Two types of cells are used, diaphragm cells and mercury cathode cells. Both types of cells have
been divided to keep the anode compartment separated from the cathode compartment in order
to prevent the reaction between chlorine and caustic soda formed respectively in anode and
cathode compartments.

7. DIAPHRAGM CELLS- usually manufactured of asbestos, allows a flow of
brine from the anode to cathode but separates the chlorine and hydrogen gas spaces.
As hydrogen ions are discharged, hydroxide ions accumulate in the cathode compartment with the
aqueous sodium ions to produce sodium hydroxide. Back migration of the hydroxide ions from the
cathode to anode is prevented by the velocity of the liquid flow from one compartment to the other.
The chlorine formed at the anodes rises through the brine into a space formed by the cell's cover.
 Two types:
(a) Submerged cells (b) Dry diaphragm cells
In these cells cathodes remain
submerged. The liquid in the cathode
compartment is at : low-level in these
cells in order to prevent the back
Brine flow of OH ions by diffusion.
In these cells electrolysis starts with dry
cathode compartment or empty cathode
compartment.
Example-Hooker cells and
Townsend cells
Examples- Vorce, Nelson, Gibbs cells
Note- graphite is used as anode ( universally )

9. MERCURY CATHODE CELLS-
• The method of producing chlorine and causticsoda in an electrolytic cell with a mercury cathode.
• THE CASTNER KELLNER CELL
In the mercury cell process, sodium forms an amalgam (a ‘mixture’ of two metals) with the mercury at the
cathode. This amalgam reacts with the water in a separate reactor called a decomposer where hydrogen
gas and caustic soda solution at 50% are produced.

10. LIME SODA PROCESS FOR THE
MANUFACTURE OF CAUSTIC SODA
In this process caustic soda is formed by the reaction between sodium carbonate
and milk of lime. The lime soda process produces lower grade caustic soda suitable
for some applications. Treatment of Trona ore (a mixture of sodium carbonate and
sodium bicarbonate) with lime is used to produce sodium hydroxide.

11. CHLORINE GAS
• Chemical formula Cl2
• Chlorine gas can be pressurized and cooled to change it into a liquid so that it can be shipped and stored. Chlorine
gas can be recognized by its pungent, irritating odor, which is like the odor of bleach.
• Chlorine, which is obtained as a byproduct in the manufacture of caustic soda, finds applications in paper and
pulp, solvents, explosives, plastics, pesticides, sanitation and various chemical industries.
DEACON'S METHOD FOR THE MANUFACTURE OF
CHLORINE
• The process that produces chlorine gas from gaseous hydrogen chloride in the presence of transition metal oxide
as an efficient catalyst at moderate temperature and without the volatilization of the catalyst is known as Deacon’s
process.
• In 1868, Deacon developed a process by which chlorine is produced by oxidation of gaseous HCl with O2 in the
presence of a Cucl2catalyst. The chemical equation for this process is given below:
HCl(g)+14O2−→−−−−−Cucl2,723K12H2O(g)+12Cl2(g) HCl(g)+14O2→Cucl2,723K12H2O(g)+12Cl2(g).
This an overall exothermic process and also fast.

12. • The process consists of two-step:
A chloridizing step in which the HCl is contacted with the catalyst at the elevated
temperature. In this step, there is a conversion of transition metal oxide to transition
metal chloride with the elimination of water.
An oxidation step in which the transition metal chloride formed in step 1 is contacted with
the source of oxygen. And there is an evolution of chlorine gas and transition metal
chloride is reconverted to transition metal oxide.
Several problems are associated with Deacon’s process. The temperature of the process
reduces the equilibrium constant for the conversion of the HCl thereby reducing the yield.
And also at elevated temperature above 675∘K, the catalyst’s activity rapidly decreases,
due to volatilization of the cucl2.

14. • Final evaporation - The 50% sodium is concentrated in huge cast iron pots on open fire. About 99%
water is removed and molten caustic soda is formed. The final temperature is 500-600°C. These
pots have now been replaced by Dowtherm heated evaporators for caustic evaporation above 50%.
Another method of dehydrating 50% caustic soda is the precipitation of sodium hydroxide
monohydrate which contains much less water than the original solution.
• Purification of caustic soda- 50% caustic soda solution still contains impurities such as colloidal
iron, NaCl and NaCIO. Iron is removed by treating caustic with 1% by weight of 300 mesh CaCO;
and filtering the resulting mixture through a filter on a CaCO3 per coat.

15.  Sodium carbonate, Na2CO3·10H2O (also known as washing soda, soda ash and soda crystals) is
the inorganic compound with the formula Na2CO3 .
 Sodium carbonate or soda ash, has widely been used in soap, sugar, glass, drugs, dyes, paper,
ceramics, textiles, metallurgy, chemicals, petroleum, leather, water softening industries etc.
Manufacture –
Soda ash, commonly known as sodium carbonate, is manufactured by making use of the following
processes.
(1) Leblanc process. (2) Solvay's ammonia-soda process.
(3) Dual process (modified Solvay's process). (4) Electrolytic process.

16. Raw material used- NaCl (common salt),sulphuric acid, limestone and coke

17.  The Solvay process or ammonia-soda process is the major industrial process for the production of sodium
carbonate(soda ash, Na2CO3). The ingredients for this are readily available and inexpensive: salt brine (from inland
sources or from the sea) and limestone(from quarries). This method superseded the Leblanc process.
 In Solvay process, also known as ammonia soda process, carbon dioxide is passed through a brine solution
(containing about 28 % NaCl) which is saturated with ammonia to form sodium carbonate.
2NH3​+H2​O+CO2​→(NH4​)2​CO3​
(NH4​)2​CO3​+H2​O+CO2​→2NH4​HCO3​
NH4​HCO3​+NaCl→NaHCO3​↓+NH4​Cl
The precipitate of sodium bicarbonate is filtered, dried and ignited to form sodium carbonate.
2NaHCO3​heat​Na2​CO3​+CO2​+H2​O
The carbon dioxide required for the reaction can be obtained by heating limestone (calcium carbonate) to 1300 K in
a lime kiln. Lime dissolves in water to form calcium hydroxide which is then transferred to the ammonia recovery
tower.
CaCO3​heat​CaO+CO2​
CaO+H2​O→Ca(OH)2​
Ammonia required for the process can be prepared by heating ammonium chloride with calcium hydroxide.
2NH4​Cl+Ca(OH)2​2NH3​+CaCl2​+H2​O
Hence, the only byproduct of the reaction is calcium chloride.

18.  Advantages of Solvay process,
(1) Low grade brine can be used. (2) Consumes less electric power than dual process. (3) Uses less raw
material. (4) Less corrosion problems. (5) No co-product to dispose off (6) Does not require NH3 plant
investment.
 (b) Disadvantages of, Solvay process
(1) High salt consumption than dual process. (2) Sodium and chloride radicals of salt are fully utilize in
dual process, giving NH4Cl us a co-product. It requires less investment than in NH3 recovery unit. (3)
More steam consumption than dual process. (4) The dual process has no efficient problem. (5) Co-
product disposal is no problem; in dual process NH4Cl is used as a fertilizer.

19.  The raw materials used in this process arc crystalline salt, fuel and ammonia. In this process NH4CI is
obtained as a co-product and it differs from Solvay process in that it does not recycle ammonia.
 The liquor from the carbonation system, containing ammonium chloride, unreacted salt and traces of
carbonate is ammoniated in an ammonia absorber. The ammoniated liquor is mixed with a bed of washed
salt in a salt dissolver. The exit liquid from the dissolver, saturated with salt, is gradually cooled in a
refrigerating tank continuous unit at 0'C to crystallize ammonium chloride. The slurry containing ammonium
chloride is thickened and ammonium chloride is centrifuged and dried.
 The liquid obtained after the separation of crystals of NH4CI is recycled to the carbonation towers placed in
series. The rich.CO3 obtained from the calciner or soda ash plant is passed through the bottom in the
carbonation vessels, which are provided with cooling coils. Sodium bicarbonate is formed. The growth of
crystals of NaHCO3 is controlled by controlling the temperature at the ends and at the middle of the tower
as in the Solvay process. Sodium bicarbonate. is thickened in a thickener and centrifuged. It is then
converted to soda ash.